Porous silica powders, with ordered porosity in the nanometer scale, have utility for catalysis, gas separation and high surface area supports for self-assembled monolayer films. Mesoporous micro-bubbles in particular, have applications in separations, thermal barriers and micro-encapsulation for drug delivery.
Micron-sized bubbles composed of solid silica walls are commercially available and are used as fillers and within reflective paint for highway signs. U.S. Pat. No. 2,797,201 (Standard Oil Co., Ohio) describes hollow glass spheres with solid walls, by spray drying liquid alkali metal silicates containing a blowing agent. Sizes range from 50-300 .mu.m. Because these products are not porous, they are not useful for catalysis, and gas separation.
Other formation techniques for mesoporous powders and films, discussed in the literature, involve slow growth from supersaturated solutions for several hours to one week. The previous methods are based on a precipitation processes in which dissolved silica co-precipitates with the surfactant micelles to form a mesoporous structure and typically involve heating the reactants in an autoclave for several hours to a week. A disadvantage of these methods is that there is no control over particle size and/or shape. Filtration, often tedious because of small particle size, is required to separate the solution from the mesoporous particles.
Work described in U.S. Pat. Nos. 5,264,203, 5,098,684, 5,102,643, and 5,238,676 shows mesoporous powder formation by in-situ solution-phase precipitation, which again requires substantial time from a minimum of about 1.25 hour to about 168 hour to obtain precipitated powders.
Huo et al., Chem. Mater. 1994, 6, 1176, discussed a method for producing mesoporous silica by an acid route. Tetraethoxysilane (TEOS) was added to a dilute aqueous solution of cetyltrimethyl ammonium chloride (CTAC) and HCl. The solution composition on a mole basis was: TEOS 1.0; CTAC 0.12; HCl 9.2; water 130. After .about.30 min of stirring at room temperature, particles precipitated and were filtered from the remaining solution. Again, a significant amount of time (30 min) is needed to obtain precipitation of particles from the solution phase.
Tanev, P. T.; Pinnavaia, T. J.; Science, 1996, 271, 1267 used surfactant vesicles to template silica vesicles in a reaction mixture. The reaction mixture was vigorously stirred at ambient temperature for 18 hours to obtain the templated lamellar product with vesicular morphology, denoted MSU-V.
The method discussed by both Kresge, C. T., et al., Nature 1992, 359, 710; and Beck, J. S., et al., J. Am. Chem. Soc. 1992, 114, 10834 involves a slow growth, or co-precipitation, of silica and surfactant micelles over a period of 4 hours to 144 hours (5 days). Beck, J. S.; Hellring, S. D.; Vartuli, J. C. Abstract # COLL-311, ACS National Meeting, April 13-17, San Francisco, Calif., 1997, further indicate that 1700 m.sup.2 /g is presently an upper limit of surface area.
Porous silica films have applications in catalysis, environmental remediation, energy storage, thermal barriers and energy storage. Porous silica films, in particular, are potentially useful as low dielectric constant interlayers in semiconductor devices, as low dielectric constant coatings on fibers and other structures, and in structured catalytic supports. Porous silica films produced by previous methods can be divided between random, gel-like silica films, and surfactant-templated films in which the pores are within a hexagonal lattice, with the characteristic pore diameter defined by the surfactant micelle.
Previous work resulting in mesoporous membranes from surfactant-templated powders and structures by in-situ solution-phase precipitation has been described in co-pending U.S. patent application Ser. No. 08/344,330. In-situ solution-phase precipitation requires substantial time from about 4 hours to 1 week to form a mesoporous membrane or film.
Hrubesh, L. W.; Poco, J. F., J. of Non-Cryst. Solids 1995, vol 188, p. 46 applied "aerogel" technology to produce high-porosity films with random porosity. In the aerogel synthesis route, a hydrolyzed silicon-alkoxide solution is metered onto a spinning substrate. To avoid drying, the spinning apparatus is in an atmosphere saturated with solvent vapor. The spinner is stopped with a brake, and the retained spinning solution gels within a few minutes. The gel-coated substrate is immersed in solvent and subsequently dried under supercritical conditions.
Smith et al. (Smith, D. M.; Anderson, J.; Cho, C. C.; Gnade, B. E., Mat. Res. Soc. Symp. Proc. 1995, 371, 261, and Smith, D. M.; Anderson, J.; Cho, C. C.; Johnston, G. P.; Jeng, S. P., Mat. Res. Soc. Symp. Proc. 1995, 381, 261) applied "xerogel" technology as an alternative to aerogels. Here, the spin-cast silica sol-gel film is aged, washed and solvent exchanged, silated with a trimethylchlorosilane solution in heptane, and dried. In contrast to the aerogel process, the film is dried at ambient pressure. The aging and chemical treatment minimizes pore shrinkage during drying and makes the film hydrophobic, but the film becomes hydrophilic on heat-treatment, unless done in a forming gas environment.
Both techniques for spin-casting (1) aerogel and (2) xerogel films are complicated by the fact that spinning must be performed in solvent-saturated atmospheres (requiring explosion proofing) to avoid premature drying of the film.
In other work on mesoporous silica films, Ogawa (Ogawa, M., J. Am. Chem. Soc. 1994, 116, 7941) fabricated spin-cast silica-CTAB films. Ogawa used a CTAB/TMOS mole ratio of 0.40 in a solution that avoided gelation or precipitation and produced films that were lamellar, containing alternating layers of silica and bilayers of CTAB, and therefore not calcinable; surfactant can not be removed without degradation of the film structure. Accordingly, Ogawa did not calcine his silica films. Although Ogawa noted that rapid evaporation was essential for the formation of highly-ordered, lamellar CTAB-silica composites, those composites would not be expected to be stable to calcination, and would also not contain useful pore structures.
Further work by Ogawa (M. Ogawa, A SIMPLE SOL-GEL ROUTE FOR THE PREPARATION OF SILICA-SURFACTANT MESOSTRUCTURED MATERIALS, Chem. Commun., 1996, 1149-1150) was with a CTAC/TMOS ratio of 0.25. However, he used a substoichiometric ratio of water to silica (TMOS) of 2 (stoichiometric ratio of water to silica is 4). Ogawa's product, before calcination, has the 100, 110 and 200 reflections in the XRD pattern corresponding to a hexagonal structure. However, no information is given on calcined films in which the surfactant has been removed. It is inferred that Ogawa's product is unstable against calcination.
Porous silica fibers, with ordered porosity in the nanometer scale, have potential applications in catalysis, environmental remediation, thermal insulation and chemical sensors. Nanoporous or mesoporous fibers using the previously described sol-gel methods and stable against calcination have not been reported.
In the previous methods in the literature, there is no direct means for controlling particle size or pore volume fraction in powder, films or fibers.
Accordingly, there remains a need for mesoporous products having well defined morphology on both the nanometer scale (1-20 nm) (solid silica and pores) and the micrometer scale (0.1 .mu.m-100 .mu.m) (the characteristic dimension of the mesoporous product), and a method for making them in less time and without the need for filtration. Where spin-casting is done, there remains a need for a straight-forward method for producing mesoporous film(s) without supercritical drying, aging, silation of the film(s), or controlled gas environments.